FIG. 1 is a cross-sectional view of the currently existing auxiliary mesh type starter. As shown in FIG. 1, the auxiliary mesh type starter which applies a starting torque to an engine has the following structure: a speed reducer 2 is adapted to decelerate the rotational torque of an armature 11 in a motor 1 and to increase rotational torques, and an isolator 4 is mounted on a driving gear 6 of an output shaft 5 and is driven by the motor 1 to rotate.
Referring to FIG. 2, an auxiliary mesh type starter comprises a motor 1, an electromagnetic switch 3 and relays 12. In the auxiliary mesh type starter, the electromagnetic switch 3 comprises an attracting coil 36, a holding coil 37 (the holding coil 37 and the attracting coil 36 are arranged on a coil frame 43), an solenoid body 7 circumferentially surrounds the outer sides of the attracting coil 36 and the holding coil 37 and constitutes a portion of a magnetic circuit, a stop seat 16 is arranged at the rear end parts of the attracting coil 36 and the holding coil 37 and constitutes a portion of the magnetic circuit, a plunger 8 is arranged on the inner circumferences of the attracting coil 36 and the holding coil 37 and is capable of sliding in an axial direction, a return spring 14 applies a return force to the plunger 8, and a movable contact point 17 is mounted at the rear end of the plunger 8, and a pair of stationary contact points 30a and 30b are arranged relative to the movable contact point 17 and are connected to the external wiring. An electromagnetic attraction force in the B-direction as shown in FIG. 1 is generated in the plunger 8 by energizing the attracting coil 36 and the holding coil 37 of the electromagnetic switch 3. Owing to the electromagnetic attraction force, the upper end of a shift fork 9 in cooperation with the plunger 8 moves towards the right direction (indicated by arrow B) and the lower end thereof moves towards the left direction (indicated by arrow A) as shown in FIG. 1. Thus, a force is applied to the isolator 4 and the driving gear 6 on the output shaft 5 to cause them to move towards the left direction (indicated by arrow A) as shown in FIG. 1, then the driving gear 6 moves towards such a direction that it is going to mesh with a flywheel gear ring 10 of the engine.
Also referring to FIG. 2, the positive terminal of a storage battery 13 is connected to a terminal 18 of the electromagnetic switch 3, and the negative terminal is grounded or connected to a terminal 31 of the motor 1. The relay 12 which switches on/off a terminal 50 of the electromagnetic switch 3 includes: a terminal 50c connected to the storage battery 13, a contact point 32 connected to the terminal 18, and a coil 34 for controlling the contact point 32 and a contact point 33. The terminal 50c located at one end of the coil 34 is connected via a key switch 35 to the positive terminal of the storage battery 13. The other end of the coil 34 is grounded or connected to the negative terminal 31.
In FIG. 2, if the key switch 35 is closed to start the engine, then the coil 34 of the relay 12 is energized to form a closed circuit between the contact point 32 and the contact point 33, and the storage battery 13 energizes the motor 1 via the attracting coil 36 of the electromagnetic switch 3 while energizing the holding coil 37. The two energized coils generate an attraction force in the plunger 8, so that the plunger 8 compresses the return spring 14 while moving in the B-direction, and the driving gear 6 moves towards one side of the flywheel gear ring 10 via the shift fork 9 (i.e., in the A-direction).
At this time, if the driving gear 6 smoothly meshes with the flywheel gear ring 10, then the plunger 8 further moves until it contacts the end face of an arresting disc 16, the movable contact point 17 comes into contact with the two stationary contact points 30a and 30b, the motor 1 is directly energized by the storage battery 13 to generate a usual rotational torque, and the driving gear 6 drives the flywheel gear ring 10 to rotate, thereby applying a starting torque to the engine. When the movable and stationary contact points are in contact with each other, the potentials of the terminal 50 and the terminal 19 are substantially the same, so no electric current flows through the attracting coil 36, and the plunger 8 is kept in the position where it contacts the end face of the arresting disc 16 only with the holding force generated by the holding coil 37.
After starting, if the key switch 35 is disconnected, then the coil 34 is not energized, a circuit break occurs between the contact point 32 and the contact point 33, and no voltage is applied to the terminal 50. So, the holding force generated by the holding coil 37 disappears, the plunger 8 returns to the state as shown in FIG. 1 with the aid of the spring force generated by the return spring 14, and, partly with the aid of the shift fork 9 cooperating with the plunger 8, the driving gear 6 breaks away from the flywheel gear ring 10. Meanwhile, the movable contact point 17 also returns to the state as shown in FIG. 1, thereby stopping energizing the motor.
As can be seen from FIG. 2, the coil of the electromagnetic switch 3 consists of the attracting coil 36 and the holding coil 37. The numbers of turns of the two coils are substantially equal, their head ends are connected together, and the tail end of the attracting coil 36 is connected to the power supply terminal (also the output terminal of the main contact point of the electromagnetic switch 3) of a DC motor, and the tail end of the holding coil 37 is kept grounded.
In addition, the attracting coil 36 of the electromagnetic switch 3 has low resistance, which is typically about 100 milliohms or so. In this way, the starter can turn slowly at a low torque before the closure of the main contact point of the electromagnetic switch 3, so that when the driving gear 6 is pushed against the end face of the flywheel gear ring 10, it can rotate slowly so that it is not pushed against the gear and then meshes with the flywheel gear ring 10; only after it meshes with the flywheel gear ring 10, will the main contact point of the electromagnetic switch 3 be closed (i.e., will the movable contact point 17 come into contact with the two stationary contact points 30a and 30b), will a large current flow through the motor 1, and will a high torque be output from the starter, thereby avoiding a gear milling failure in the starter. Therefore, such starters are also called flexible mesh starters.
Such conventional auxiliary mesh type starters have the following problems:
(1) Since the head end of the attracting coil and the head end of the holding coil of the electromagnetic switch in this type of starter are connected together, in order to guarantee the reliable power off of the electromagnetic switch, the attracting coil and the holding coil of the electromagnetic switch must have substantially the same effective number of turns and, in the meanwhile, the holding coil must not have too few turns. This means that the attracting coil must also have quite a few turns. Although the starter can be enabled to rotate slowly before the closure of the main contact point of the electromagnetic switch by a method which comprises appropriately increasing the coil diameter of the attracting coil and reducing the number of turns of the attracting coil, the number of turns of the attracting coil cannot be reduced sharply, otherwise the number of turns of the holding coil has to be reduced drastically with an eye to the reliable power off of the electromagnetic switch. Because of the limited torque for the flexible meshing of this type of starter, in some cases, the driving gear cannot mesh with the flywheel gear ring and accordingly is pushed against the gear. As a consequence, the driving gear cannot mesh with the flywheel gear ring, thus the attracting coil is forced to be energized for a long time while a relatively large current flows through the coil, so the electromagnetic switch is prone to failure.
(2) Because a relatively large current flows through the attracting coil which has quite a few turns, a large electromagnetic force is generated by the electromagnetic switch and accordingly the driving gear applies a too large acting force to the end face of the flywheel gear ring, thereby badly damaging the end face of the flywheel gear ring; furthermore, since the driving gear applies a too large acting force to the end face of the flywheel gear ring, the driving gear of the starter is liable to be pushed against the gear, and if so, the transmission of the driving gear will be impeded by a high drag torque, and the fault that the electromagnetic switch is burnt out will easily occur as the driving gear is pushed against the gear for a long time.
(3) In order to ensure that a sufficiently large current flows through the attracting coil, the attracting coil has not many turns, thus the holding coil has not many turns, too, the coil has a higher current density, and the starter works for a long time, keeping elevating the temperatures of the coils too rapidly. Due to heat conduction, the attracting coil has a too high temperature, the starter has a too small braking torque for flexible meshing when it starts up again, then the faults of pushing against the gear and of burning out the electromagnetic switch would easily occur in the starter. If a method comprising increasing the coil diameter of the holding coil and rewinding it is employed for reducing the current density of the holding coil, such coil assembly is poor in winding process and the cost of the holding coil is high.
(4) In some abnormal conditions, e.g., when the flywheel gear ring and the driving gear do not match properly, the main contact point of the electromagnetic switch cannot be closed, then the attracting coil is compelled to have a large current flowing through it for a long time, so the fault of burning out would occur to the electromagnetic switch easily.